Batteries are devices that covert chemical energy within its material constituents into electrical energy. Three structural components are required for such transfer: the anode, or negative electrode material, which is oxidized, the electrolyte, which serves as a conductor of charged ions or electrons, and the cathode, or positive electrode, which is reduced. Chemical reactions at the electrodes produce an electronic current that can be made to flow through an appliance connected to the battery. In a rechargeable (or “secondary”) battery, once the reactions have run their course, they can be reversed by the action of a power supply or charger. Desirable characteristics of secondary batteries include high power density, high discharge rates, flat discharge curves, and a good low-temperature performance.
The process of transferring electrons from one material to another involves a redox reaction in which one material is reduced (thereby acquiring electrons) and another oxidized (thereby releasing electrons). The choice of materials used to form a battery is complicated, and is affected by the chemistry of the redox reaction, as well as by concerns relating to battery size, weight and cost, by polarization, and by complications caused by reactivity with other components (www.hrst.mit.edu/hrs/materials/public/Tutorial_solid_state_batteries.htm). The anode should be a good reducing agent, exhibit good conductivity and stability, and be easy and cheap to produce. Metals are most commonly used as anodes. The lightest metal, lithium, has most often been chosen. Lithium is also very electropositive (so that combined with an electronegative cathode, a large electromotive force will result). The electrolyte must be an insulator of electrons to prevent the battery cell from self-discharging. It also serves as a charge separator of the two electrodes. At the same time it must be an ionic conductor. In typical batteries, the electrolyte is composed of a liquid such as water having dissolved salts, acids or alkalis. Cathode materials are especially important for the quality of the battery: the available energy of the battery is proportional to the cathode size and directly related to many other characteristics.
The classical secondary battery contains two reversible solid-reactant electrodes and a liquid electrolyte: S−/L/S+. The Plante lead-acid cell commonly employed in car batteries is a typical example: Pb/H2SO4/PbO2. During discharge, the so-called double sulphate reaction occurs: Pb+PbO2+2H2SO4→2PbSO4+2H2O (both electrodes are converted into lead sulphate) (see Visco et al., U.S. Pat. No. 5,516,598. The processes at the two electrodes involve dissolution and precipitation, as opposed to solid-state ion transport or film formation. The cadmium-nickel battery used for heavy-duty tasks and emergency (standby) power is a second example of a classical secondary battery. Sealed cadmium-nickel batteries are widely used for smaller appliances, portable tools, electronic and photographic equipment, memory back-up etc. The basic electrochemistry of discharge is: 2NiOOH+2H2O+Cd→2Ni(OH)2+Cd(OH)2. In this discharge reaction, trivalent nickel hydroxide is reduced to divalent nickel hydroxide through the consumption of water, and metallic cadmium is oxidized into cadmium hydroxide. Liquid electrolyte batteries are disclosed by Ventura et al. U.S. Pat. No. 5,731,104; Ventura et al., U.S. Pat. No. 6,015,638.
In order to identify battery systems that might provide couple electrochemical properties with smaller size or weight, researchers have long sought to define suitable solid-state battery systems. Solid electrolytes are of particular interest for secondary batteries and for fuel cells (www.web.mit.edu/newsoffice/tt/1998/apr29/battery.html; Munshi M Z A (ed.), “Handbook Of Solid-state Batteries And Capacitors,” Intermedics Inc., USA, (1995). The first such system (a sodium/sodium-β-alumina/sulfur battery) was developed in 1967, and used a polycrystalline ceramic (b-alumina) to conduct sodium ions at temperatures above 350° C. Unresolved problems, including high failure rates, the short lifetime of the ceramic electrolyte and lack of reproducibility have limited the utility of such batteries.
Reversible lithium solid-state batteries have been developed in which an anode of metallic lithium is separated from the cathode (an intercalation compound such as titanium disulfide or lithium cobalt oxide) by a glass electrolyte. One advantage of this type of battery is that the overall resistance does not increase with discharge. The electromotive force (emf) is approximately 2V (this emf can vary widely with cathode materials) with only a slight and continuous decrease with loss of capacity. This contrasts with conventional batteries, which experience an abrupt loss of voltage without warning upon depletion. For example, such batteries may contain an anode having lithium between graphitic coke layers, an amorphous polymer electrolyte (such as LiCoO2/El/carbon; LiNiO2/El/carbon; or LiNi0.2CoO0.8/El/carbon). Although lithium cobalt oxide (CoO2) is similar to titanium sulfide (TiS2)in structure and behaviour, it is much more oxidizing than TiS2, and thus produces a cell having an emf of about 3.5V, almost three times as much as a nickel-cadmium or nickel-hydride battery. Secondary lithium batteries are discussed by Iwamoto et al. (U.S. Pat. No. 5,677,081) and Takada et al. (U.S. Pat. No. 6,165,646). Additional information relevant to efforts to define improved battery systems is disclosed in WO9719481, WO09514311; WO09533863; WO09507555; WO09413024; WO09120105, and in Abraham et al., U.S. Pat. Nos. 5,510,209 and 5,491,041.
Polymeric compounds have also been used in batteries (see, e.g., Narang et al., U.S. Pat. Nos. 5,998,559 and 5,633,098, which describe the formation of batteries having a single-ion electrolyte through the use of functionalized polysiloxanes, polymethacrylates and poly(alkylene oxides). Takeuchi et al. (U.S. Pat. Nos. 5,874,184 and 5,665,490) discloses a battery having a solid polymer electrolyte comprising a composite of a polymeric component (see also, Narang et al. U.S. Pat. No. 5,548,055. Polymeric compounds used in batteries are also discussed by Llompart, S. et al., “Oxygen-Regeneration of Discharged Manganese Dioxide Electrode II-General Phenomena Observed on Electro-Deposited Layer Electrodes and Membrane Electrodes,” J. Electrochem. Soc., Vol. 138 (No. 3), page 665 (1991); see also Takeuchi, et al. (U.S. Pat. No. 5,597,661).
Despite such efforts, a need remained to find materials with high conductivity at ambient temperatures. High molecular weight polyethylene oxide hosts with lithium salts, polyvinyl ether hosts, and electrolytes formed by trapping a low molecular weight liquid solution of a lithium salt in an aprotic organic solvent, within the polymer matrix of a high molecular weight material have all been explored in a search for suitable materials.
In particular, conventional, liquid electrolyte batteries have significant drawbacks, particularly for electronics. These disadvantages include weight, large size, and the possibility that the electrolyte might leak. In addition, as computers and mobile phones have become smaller and faster, their demands for battery power have increased. Although solid-state batteries typically have a lower-power density than conventional batteries, they exhibit improved energy density, are re easily miniaturized (even as thin films), and cannot leak. In addition they are long-lived and their performance does not markedly change at high or low temperatures. For these reasons, solid-state batteries are well suited for electronic devices. The present configurations of solid-state battery have a liquid or gel electrolyte between the anode and cathode. Unfortunately, such battery configurations lead to problems involving electrolyte loss and decreased performance over time. A need therefore exists for improved solid-state battery systems. The present invention addresses this and other needs.